混合型快速断路器

New hybrid circuit breaker/current limiter with serial and parallel commutation assistance Brevet n° 03 293 050.5 Ronan BESREST - CAPSIM Pierre SELLIER - TECHNICATOME groupe AREVA Claudio ZIMMERMAN - EPFL Abstract - This paper presents a new hybrid circuit breaker topology based on the use of serial and parallel semiconductors that allows to increase the supply voltage and default current with no arc, and to benefit from a current limitation function. A concrete marine application, with 6.6 kV AC breakers, have been studied and simulated for the DGA (French defence procurement agency). The whole constraints have been shared between the different components in order to optimise the system. This new circuit breaker shows interesting performances and new functionalities that should be able to give additional margins on network components design. INTRODUCTION Medium voltage circuit breakers are classically electromechanical device. In such systems, the circuit breaking is realized with the extinction of an electrical arc when the breaker opens. The latest progresses in power electronics make realistic to replace the mechanical part of a circuit breaker by semiconductors, in order to get fast and long life systems. Such static circuit breakers have been tested but one major disadvantage is the thermal losses due to the voltage drops. Hybrid breakers based on a fast mechanical switch and semiconductors connected in parallel are also studied in order to get all the advantages of semiconductors and avoid disadvantages of conduction losses. Fig 1: basic hybrid AC breaker topology When the mechanical switch opens, a small arc voltage deflects the current in the semiconductors branch which finally interrupts the fault current. IGCT (integrated gate turn off thyristor) have been used successfully in such medium voltage applications [1]. But limitation in off state blocking voltage and current level are always problematic with static components especially in our focused application (6.6 kV). The presented solution based on serial IGCT cells and parallel thyristor commutation assistance modules allows the use of hybrid solutions in new fields of application. Thanks to these modules it is possible to reach greater power and voltage levels in power breaking applications. APPLICATION CONTEXT This study was realised on the requirement of the DGA (Délégation Générale pour l’Armement – (French defence procurement agency) in the context of the All Electric Ship Program. The final aim of this circuit breaker is to be integrated in a three phases 6.6 kV/60 Hz marine power plant. Fig 2: application network topology Its main function is to protect a 6.6kV/440V transformer with a rated load from 1.5 MW to IGCT 6.6 kV / 60 Hz 440 V / 60 Hz 1,5/5 MW Circuit breaker 5MW. The power sources on the network are turbo generators and diesel generators that work in parallel operation. A - Main specification of the hybrid breaker • The arc energy should be minimal or null in order to get a long life breaker with minimal maintenance. • In fault conditions, the breaker should be able to eliminate the default after a time switch delay in order to manage time selectivity for protection plan. • Thermal losses on the breaker should be minimized because in this embarked network, cooling systems must be light and reliable (natural convection). • The mechanical part should be basic because of a harsh mechanical environment. B - Voltage constraint The rated voltage is 6.6 kV, but normal voltage variations in this isolated network should be considered (+-16%). Special conditions of non synchronous coupling of the three phases have to be taken into account. As a result, the global voltage constraint is: Uppmax = 12.5 kV. C - Current constraint The protected load is from 1.5 to 5 MW. The current constraints are: The maximal current rising speed is 20 kA/ms and occurs on a medium short circuits case. If we consider the classical hybrid AC breaker topology (fig 1) and the actual available semiconductors properties, the number of needed serial IGCT is 4 in this application. They are necessary to withstand a turn off voltage of 12.5 kV with safety margins. The arc voltage used to deflect the current [1] should be greater than 16 V and should be reached in a short time in order to minimize the arc energy. Considering these values, increasing the application voltage to 6.6 kV leads to a significantly fast mechanical switch. The needed speed and voltage to withstand in our context for a non energetic arc cannot be reached easily with actual mechanical switch basic technologies. PRESENTATION OF THE SOLUTION A new topology is proposed in order to manage the voltage and current constraint. The main idea is to share the voltage constraint between the fast mechanical switch and a serial semiconductors cell. The aim is to get no arc using a basic fast mechanical switch and allowing minimal conduction thermal losses. This topology is build following a modular and functional approach. A- Circuit breaker modules The different functional subsets of this new hybrid circuit breaker are shown below: Fig 3: novel breaker topology Module 1: Main conduction module It has to conduct the current with no losses. After the break, it has to withstand the complete supply voltage of 6.6 kV. This module is based on a fast mechanical contact. Module 2a: Deflection serial assistance This block is designed in order to increase the voltage in the main branch and to enable the current deflection in the parallel branch. This module is composed with two IGCT cells put in an anti-parallel position. These cells are modern versions of the GTO cells. It is the best device today for this application because it is able to conduct high currents (>4000A) with about 1V voltage drop, and to block a high off state voltage (4500V). In parallel of these IGCT cells, a Metal Oxyde Variator (MOV) has been added. The threshold voltage of the MOV is lower (2.8 kV) than the supply voltage value. When the semiconductors are turned off, the MOV provides a voltage, and the current begins to deflect in the parallel branch. With this module, it is not necessary to No load 0.7 A 1.5 MW rated load 150 A 5 MW rated load 500 A Low voltage short circuit 2340 A Medium voltage short circuit 100 kA 4 Energy storage and discharge 2b Parallel deflection arc limitation Main conduction module 3 Current limitation 2a Deflection serial assistance Lc C R1 Thyristors MOV IGCT S La use an arc voltage to deflect the current any more. Module 2b: Parallel deflection and arc limitation Module 2a deflects the current in the parallel branch through the module 2B path. This module conducts the current and enables module 1 to turn off without any electrical arc (when the current is null). It is composed with to 2 anti- parallel cells of 3 serial 8.2kV thyristors. These thyristors are finally used to cut the current. Module 3: Current limitation module This module is used to limit the current and cut it after it has passed by cell 2b. It is composed with inductive impedances. These inductances are needed to protect the circuit breaker during the short circuit current circulation in the parallel branch, especially if a time switch delay is needed. Module 4: Energy storage and discharge The short circuit current inductive energy is stored in the capacitor module, before being discharged in the parallel resistor. The chosen capacitor can be film metallized capacitors for pulse discharges. These capacitors have a very high energy density (up to 1kJ/L). Unfortunately, these capacitors are unidirectional in voltage. Consequently, a Graetz bridge is used to maintain a positive voltage on the capacitor. B- Circuit breaker basic working sequence When the circuit breaker is on, the current goes through the main conducting module 1, as the IGCT are on. When a fault occurs, the IGCTs are turned off, and a voltage of 2.8 kV appears in the main branch between the MOV terminals. The parallel branch thyristor is then positively biased and turned on, so the current is deflected from the main path to the parallel path. Then the mechanical switch opens with no current, so without arc. The capacitor C is used to store the inductive energy of the electrical network. The global line inductances associated with the capacitor represent an oscillator circuit. This oscillator circuit creates a current pulsation that allows to get a fast null current and to turn off the thyristors. After the break, the capacitor discharges down through the resistor. Two variant topologies are possible: - Without auxiliary inductance La - for an immediate switch of breaker In this circuit, the current is cut off after half a sine wave period at a high frequency pulsation in a short time and under limited curent. - With auxiliary inductance La - for a switch time delay breaker with current limitation With this variant circuit, an auxiliary inductance is settled between the thrystors cell and the final breaker therminals. The idea is to be able to maintain a default current in the parrallel branch during several sine wave period without overpassing the capacitor voltage limit. This is possible if the current is limited by a sufficient impedance, the auxiliary inductance (La). RESULTS These results are based on numerical simulations and analytical calculations on a complete circuit. The variable network inductance has been choosen as a worst case (Ld=280uH). The real components are choosen in manufacturer catalogs after a complete design. The following switch off sequences are based on a medium voltage short circuit stess simulated on the 6.6kV transformer terminals (fig 1). A – Circuit breaker without auxiliary inductance Switch off sequence: Fig 4: switch off without auxiliary inductance La Capacitor voltage (V) Mechanical switch voltage (V) Current in the network (A) Current in the switch (A) When the short circuit measured current reaches 1500A, an order is sent to the mechanical switch for its opening. It takes almost 100µs before the effective opening (mechanical delay). The IGCT cell is turned off during this delay but not too early in order to limit the rising voltage on the capacitor. Then, the current in the main branch, goes through the MOV, which saturates and applies a voltage on the thyristors branch. This voltage enables the proper thyristor to be turned on. The current is then deflected in the parallel branch, and the mechanical switch opens while the current in the main branch is null. The Lc inductance limits the rising current in the thyristor in order to protect it at turn on. The current is null, after half a period of the resonant circuit. In our particular case, it takes 600us. When the current is null in the thyristors cell, these ones are turned off. The maximum current value is lower than 3800A. Switch on sequence The MOV of the module 2a turns on when its voltage is greater than a threshold Uvo. This threshold is lower than the supply voltage Consequently, the mechanical contact has to be switched on when the supply voltage is lower than Uvo, in order that no current will go through the contact while it closes, and no arc will be produced. So this circuit breaker can switch the power on during a time period tuv each half a period of the sine wave as it is shown on the picture below: Fig 5: switch on without auxiliary inductance B - Circuit breaker with auxiliary inductance Switch off sequence: Fig 6: switch off with auxiliary inductance With the help of the auxiliary inductance, the rise in current is bounded and the time before switching off the thyristors can be longer. Indeed, the voltage of the capacitor does not reach the same value than in the former case. As the auxiliary inductance is high (>1.5mH), the LC oscillating frequency is lower and the current crosses zero after a complete supply voltage period. The limited default current can be maintained in the parallel branch for several periods. Thus it can be tuned by a variation of the delay angle of the thyristor switches. The first interest is the possible time switch delay in the breaker turn off that allows using time selectivity in the network protection plan. The second advantage is to be able to limit the default current to about 10 kA a 60 Hz the parallel branch instead of 100 kA that was the normal short circuit current value Switch on sequence: The limitation current function cell enables a soft switch on. Indeed, it is possible to activate the parallel branch first with a tuneable current level by the use of variable delay angle of the thyristor switches. This function can be used to manage the magnetisation of the transformer load, with a proper decreasing of the angle. When the minimum angle is reached, the voltage of the circuit breaker is null, and the mechanical contact can be closed +UV0 tUV Closing of the mechanical switch -UV0 Capacitor voltage (V) Mechanical switch voltage (V) Current in the network (A) Current in the switch (A) C - Additional calculations The calculation of the thermal losses shows that in rated conditions, they does not overpass 191W that enables the use of natural convection to evacuate the themal power. COMPARISON WITH EXISTING SYSTEMS A – Comparing with electromechanical solutions • The turn off time delay can be managed from 600us to several milliseconds according to the protection plan. • The short circuit current value can be managed. • The interruption is realized with no arc so the mechanical breaker is not damaged at each sequence. • The phenomena are easier to manage and more reproducible compared to the phenomena in a breaker chamber. • Modular functional concept which allows optimisation according to network architecture and application. • New functions can be added on the same equipment: – Default current limitation. – Transient current limitation (soft turn on, motor start...). These functions can deliver margins in the design of other network components. B – Comparing to hybrid/static solutions • The supply voltage is increased to 6.6kV with a basic fast mechanical switch. • The short circuit power can be higher (the initial peak current default value can reach 100 kA). With static topologies, it would lead to a huge number of serial semiconductors to withstand such a current • The thermal losses are reduces compared to static solution that allows natural convection. • It is possible to get a time switch delay for the breaker detection logic. • Current limitation functions are available. FUTURE DEVELOPPMENTS This new topology concept for medium circuit AC breakers has been tested successfully in simulation. It should be validated on real conditions. The next step in this study, which is beginning at the present time, is to develop a reduced scale prototype in order to demonstrate for each module that the constraints are withstood in every severe condition. CONCLUSIONS This new hybrid circuit breaker is a modular concept that can be used to manage severe electrical constraint on a medium voltage embarked network. It should be able to interrupt fault current with no arc. The voltage and current constraints can be shared between the components in order to fulfil the requirements. The use of serial IGCT with minimal thermal losses allows decreasing the complexity and speed of the mechanical part of the system. New functions of current limitation are available by the use of a variant topology that integrates an auxiliary inductance. Such functions could be able to give new margins in the equipment definition of the networks. Numerical simulation gave impressive results concerning interruption time and current limitation capability. This topology will be soon tested on a prototype that is developed at present time. [1] Jean-Marc Meyer - Etude et réalisation d’un disjoncteur hybride ultra rapide à base de thyristor IGCT- Thesis EPFL 2000. [2] Jungblut - Hybrider Schnellschalter mit Dioden als Kurzzeitladungsspeicher VDI Reihe 21 Nr. 269 Düsseldorf: VDI Verlag 1999. [3] Holaus - Ultra fast switches – Basic elements for future medium voltage switchgear -Thesis ETHZ No. 14375 dem Walter Holaus, 2001. [4] Steurer - Ein hybrides Schaltsystem für Mittelspannung zur strombegrenzenden Kurzschlussunterbrechung. Thesis ETHZ 2001. M. Steurer. Abstract INTRODUCTION APPLICATION CONTEXT A - Main specification of the hybrid breaker B - Voltage constraint C - Current constraint PRESENTATION OF THE SOLUTION A- Circuit breaker modules Module 1: Main conduction module Module 2a: Deflection serial assistance Module 2b: Parallel deflection and arc limitation Module 3: Current limitation module Module 4: Energy storage and discharge B- Circuit breaker basic working sequence RESULTS A – Circuit breaker without auxiliary inductance B - Circuit breaker with auxiliary inductance C - Additional calculations COMPARISON WITH EXISTING SYSTEMS A – Comparing with electromechanical solutions B – Comparing to hybrid/static solutions FUTURE DEVELOPPMENTS CONCLUSIONS Menue